This allows us to re-use existing tests (assuming the call and fields
are covered by `cln-rpc` and `cln-grpc`) to test the full roundtrip
from test over the grpc interface to the json-rpc interface and back
again.
You can switch to the grpc interface by setting the `CLN_TEST_GRPC`
environment variable to 1, but for now only very few shims are
implemented (due to the non-generated nature of LightningRpc).
